replace_history_parametert: use an unordered map to symbol_exprt

There are no ordering requirements here, so we can benefit from O(1)
amortised cost. Also, the value type is always a `symbol_exprt`, not
just a generic `exprt`.

Author: tautschnig

src/goto-instrument/contracts/utils.cpp

@@ -453,7 +453,7 @@ void add_quantified_variable(
 static void replace_history_parameter_rec(
   symbol_table_baset &symbol_table,
   exprt &expr,
-  std::map<exprt, exprt> &parameter2history,
+  std::unordered_map<exprt, symbol_exprt, irep_hash> &parameter2history,
   const source_locationt &location,
   const irep_idt &mode,
   goto_programt &history,

src/goto-instrument/contracts/utils.h

@@ -245,7 +245,7 @@ void add_quantified_variable(
 struct replace_history_parametert
 {
   exprt expression_after_replacement;
-  std::map<exprt, exprt> parameter_to_history;
+  std::unordered_map<exprt, symbol_exprt, irep_hash> parameter_to_history;
   goto_programt history_construction;
 };